Laboratory investigation of boundary condition impacts on nitrate anion exclusion in an unsaturated soil

Authors: Allred, Barry; BROWN, GLENN - Oklahoma State University; Martinez, Luis

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2015
Publication Date: 6/15/2015
Publication URL: http://handle.nal.usda.gov/10113/61406
Citation: Allred, B.J., Brown, G., Martinez, L.R. 2015. Laboratory investigation of boundary condition impacts on nitrate anion exclusion in an unsaturated soil. Soil Science Society of America Journal. 79(3):742-756.

Interpretive Summary: Nitrate is the most widespread contaminant found in ground and surface waters. Agricultural fertilizers include nitrate, often in highly soluble forms easily transported through the soil profile. In order to better assess the environmental risks of nitrate fertilizers, a better understanding is needed of the anion exclusion process governing nitrate mobility in the soil environment. Twenty-seven laboratory soil column tests were carried out to evaluate the impact of total soil wetness conditions, initial soil wetness conditions, and initial soil nitrate concentration on nitrate anion exclusion. Initial wetness conditions and the initial soil nitrate concentration affected the distance into the column to which the nitrate penetrated. In dry soil, the nitrate didn't penetrate as far, but did produce a high concentration peak at its penetration front. In wetter soils, the nitrate penetrated further into the column, but peak concentrations were suppressed. The results of this work will help better predict nitrate movement within the soil profile.

Technical Abstract: Transient unsaturated horizontal column experiments were conducted with a loam soil, under variable boundary conditions, to obtain added insight on anion exclusion processes that impact nitrate transport in soil. The boundary conditions evaluated were column inlet soil water content, initial soil water content, column inlet soil solution nitrate concentration, and the initial soil solution nitrate concentration. Results consistent with significant anion exclusion of nitrate were exhibited in all the column test results. For tests with nitrate solution injected at the column inlet, the measured soil solution nitrate concentration at the inlet was found to be 12% to 19% less than the injected nitrate concentration. For these same tests, where initial soil conditions were dry with no nitrate originally present, the nitrate concentrations at the wetting front edge were substantially greater than the injected nitrate concentration, while for initially wetter conditions, with or without nitrate originally present, the nitrate concentration front was found to extend well beyond the theoretical "piston front" separating the injected solution from displaced initial soil water. For displacement tests, in which water was injected at the inlet of a column initially wetted with a nitrate solution, the soil solution nitrate concentration front also extended beyond the piston front. Inlet nitrate excluded water content ranged from 0.03 to 0.06 and was not affected by pore water velocity, but did have a modest positive correlation with total water content (r = 0.60 and r = 0.67). The findings of this investigation provide a better overall understanding of nitrate movement through the soil profile.